@edoni,
It should work, You may look over all the inserts to make sure you put them in the correct area.
Can you give link to your site so I can see what is happening?
@dizizi,
I would think it uses the same amount of memory as it would if it was creating thumbnails or any other resizing command that you do on the site.
One nice thing is that it only does it once for each size, after that, if it sees the file already created then it does not re-create it.
I know that did not exactly answer your question but I do not know how to monitor mem usage as a script runs,
sorry